I'll pick apart my opinions on TD for this, considering it's my "main game." I'm a serious Easy/Normal score runner and casual Hard/Lunatic survival player, so I'll do my best to offer opinions on both of these.
First I'd like to say (and you may already know this) that your final product will be shaped heavily by the scoring. That is, if you plan to employ a deep and intricate scoring system that leaves a lot of room for experimentation rather than "survive to score", where NMNB is the way to score high. In the case of the latter, you have much more freedom to make the game flow however you want it to. The former seems a little bit more tricky to get right, at least as far as I see it. Take, for instance, Ten Desires. The game's stage scoring revolves around killing enemies in quick succession as soon after they come on screen as possible. After a certain amount of chaining, you begin to gain gray spirits (which are worth as much as your current PIV or point item value as well as increasing your trance meter substantially) on top of blue ones (which increase PIV). As such, ZUN has crafted the stages in a way that allows the player to abuse the system as much as possible. There's little use in having the mechanics listed above if the enemies do not appear in an abusable order. If, say, the enemies appeared far left, then far right a split second later, and kept alternating this pattern, I doubt you would even gain gray spirits as the time taken between killing each enemy is far too large. You get the idea. What you actually find is that enemies are often placed in two groups: one wave will come from the left, while another set of enemies from the right. These sets of enemies come into the stage in such a way that they can be chained and quickly killed off in order to gain the spirits they drop. More often than not, there are multiple ways of destroying the enemies whether it be as simple as choosing one wave to kill of first that you deem to be worth more for PIV or points in general, leaving the second wave and missing out on the spirits, only killing them once you've destroyed the other wave. (This is one hell of a ramble and looking back on it may not make too much sense, I'd suggest taking a look at an Easy scorerun to see what I mean about choosing which enemies to kill. If you do need more clarification about this part, just ask.)
As mentioned, the boss patterns are likely to morph with your scoring system too. In TD, a lot of the boss patterns are "shotgun-able." By shotgunning is staying directly underneath the boss making sure all of your shot hits them. This is a big thing in TD as bosses also spawn spirits in a similar way to stage enemies, the closer in proximity you are to the boss (usually maxes at just within the boss health circle) and how much damage you are doing to the boss dictates how many spirits will spawn, as far as my understanding leads me to know. Obviously the more the better, if you're scoring! ZUN specifically leaves spaces in danmaku and patterns in general for the player to milk spirits as much as is humanly possible, with it obviously being far easier on Easy mode.
Resources are also a huge aspect of TD, as they offer new and different ways of gaining spirit where you normally may not be able to. There are two fairly obvious places throughout the game you can do this - in sections with a lot of enemies you cannot kill quick enough (see Stage 4 TD) or during boss attacks that are too hard or straight up impossible to shotgun. This also expands a bit on the depth I brushed on earlier, leaving room for experimentation on where to bomb, where to suicide, where to spend trance - should it be on gaining extra resource pieces (x2 gained during trance) or to gain PIV (blue spirits go from 10 PIV to 100 PIV in trance), should I spend my resources during bosses or stages, which spellcards/attacks should I bomb/trance/suicide - you get the idea. ZUN has purposefully given the player many, many choices to pick from that may even evolve depending on how the run is going, if you miss a bomb or die accidentally your route is severely thrown off. He has also placed very obvious cash-in areas such as Stage 4 and 6 pre-boss.
Enough of the meta-score running stuff for now though, hopefully it offers an insight into my own mind regarding scoring and what makes TD fun for me. Let's get into some stuff I actually don't like about TD.
Stages 1 through to 3 are dull. It makes sense that ZUN worked his way up in enemy spawning complexity throughout the game but the first few stages truly are downright boring. This is only about the Stages however, boss fights are fine.
I do not like the invincibility during trance. It's 10 seconds of positioning myself correctly and that is it. During stages it's a bit more fun as I can focus on killing enemies as quick as possible, but during bosses you position yourself on top of them and wait for them to move half the time. It really takes away from the patterns being fun, you can go through the entire game every single time you play not having to bother dodging a certain card because you're "scoring." I much prefer Extra scoring, as the spellcard bonus is actually enough to warrant not trancing through them, as well as the amount of spirits being gain reduced significantly if you do. Spellcards in extra are worth 14m and up if I remember correctly, normal game has a couple of million at most. (not that I've ever checked the main game's spellcard bonus because it is THAT insignificant.)
These are the only two large issues I had with TD that I can even remember, I swear I had more... I blame not playing for a little bit. I'll probably post more as I think of things.
